A vehicular hydraulic brake system provided with a brake pressure control apparatus to perform anti-lock control in order to prevent road wheels from locking during brake actuation of a brake pedal and traction control in order to prevent the driving road wheels from spinning when a vehicle starts to move or increase speed is known and is disclosed in Japanese Patent Application Laid-Open No. HEI 5-507667.
An apparatus for controlling vehicle motion through use of a brake pressure control apparatus to restrain an excessive over-steer and an excessive under-steer which will occur during, for example, cornering by a braking force to one wheel or several wheels of the vehicle irrespective of depression of a brake pedal is also known and is disclosed in Japanese Patent Application Laid-Open No. HEI 2-70561.
The vehicular hydraulic brake system generally has two brake pressure circuits each of which includes two wheel brakes. The brake pressure control apparatus, which individually controls the brake pressure in the first and second wheel brakes during actuation of the brake pedal and individually supplies and controls the brake pressure in the first and second wheel brakes during non-actuation of the brake pedal, is required to perform anti-lock control, traction control and vehicle motion control.
Several other requirements are also placed on brake control systems. The brake pressure control apparatus is also required to be reduced in size, weight and cost. The brake pressure control apparatus is further required to rapidly increase the wheel brake pressure during non-actuation of the brake pedal, and to supply brake pressure from a master cylinder to the wheel brake when the brake pedal is depressed during the vehicle motion control.